Flexible circuitry made using (e.g.) polyester or polyimide film can comprise the thinnest circuit manufactured in the electronic industry. Polymer dielectrics are used with minimum thickness for heat management purposes. However, even thin organic films can have substantial resistance to thermal transfer. Organic dielectrics have a relatively low thermal conductivity (less than about 0.5 watt.multidot.m.sup.-1 .multidot.K.sup.-1). Certain other dielectrics, such as ceramics, tend to have substantially higher heat transfer characteristics, but have significant manufacturing and operating drawbacks.
______________________________________ Thermal Transfer for Certain Electronic Materials Thermal Conductivity Material (Watt m.sup.-1 K.sup.-1) ______________________________________ Aluminum 236 Polyimide 0.15 Acrylic Adhesive 0.20 Copper 398 Alumina 20 Beryllium Oxide 275 ______________________________________
Heat transfer is a very important engineering problem in modern circuit design. The reduction of thermal-mechanical stress in thermally active circuit environments is important. For example, thermal stress is one of the most serious causes of solder joint failure in application of electronic technology. With the accelerated rate of electronic technology advancements, the active electronic circuits installed in a variety of harsh environments are exposed to severe thermal stress. Such environments include airliners, automobiles, life support systems, missile guidance and control systems, lap top computers, etc. In a thermally active environment, greater thermal stresses in combination with a rigid printed circuit board promotes circuit failure. A clear need exist to increase the thermal transfer characteristics of the flexible films used in circuit board manufacture.
Conventional heat sink technology involves attaching a power dissipating active circuit to a heat sink. The circuit is commonly attached to the heat sink through a dielectric to ensure appropriate electrical isolation. The dielectric layer is selected with thermal transfer characteristics such that the power generated by the circuit can be adequately removed to ensure proper circuit function over the lifetime of the device. One important thermal transfer rate limiting barrier comprises typically the dielectric film forming the chip carrier, printed circuit board or other polymer layer mechanically supporting the power circuit, or integrated circuit chip. In large part in the past, the engineering focus on improving thermal transfer characteristics of flexible films has been directed to either reducing the thickness of the film or using a more conductive organic or mixed organic/inorganic chemical composition in the film. Thin films tend to dissipate heat more rapidly than thick films. Further, a variety of films have been manufactured with both organic and inorganic materials in the film that provide greater heat transfer characteristics to the film. However, these engineering solutions to heat transfer characteristics do not provide heat transfer characteristics optimized to the level of a metal layer.
Accordingly, a substantial need exists for forming in a flexible layer, in the form of a chip carrier, flexible printed circuit board, etc., a heat sink or a thermal transfer device that can be used in combination with an external heat sink to remove heat from an active power generating circuit. This need includes a requirement that the film portion of such a circuit construction provides heat conductance substantially greater than that present in current available polymeric films to the degree that the thermal characteristics of the film approaches metal thermal conductivity.